1. Field of the Invention
The present invention relates to a magnetic core of a compressed compact used in a coil for a power supply circuit and also relates to a method of producing the magnetic core.
2. Description of the Related Art
Choke coils are used in step-up and step-down circuits and smoothing circuits of electronic devices. The choke coil accumulates, as magnetic energy, a magnetic field generated by a current. The number of lines of magnetic force permeable through a magnetic core has a limitation. Upon reaching the limitation, even when a current supplied to the choke coil is increased, the number of lines of magnetic force passing through the magnetic core is not increased over the limitation and the accumulated magnetic energy cannot be increased any more (magnetic saturation). If relative permeability of a core material constituting the magnetic core is large, a larger number of lines of magnetic force are generated even with a small current, thus causing the magnetic saturation. Accordingly, a magnetic core made of such a core material having large relative permeability is not suitable for a choke coil used in a power supply of an electronic device in which a large current flows. For this reason, the magnetic cores used in these applications have been designed such that a gap is formed in a magnetic path to generate a demagnetizing field in a direction to reduce a magnetic field within the magnetic core, thus reducing apparent permeability (see Patent Document 1; Japanese Unexamined Patent Application Publication No. 2003-7536).
As an amorphous soft magnetic iron alloy, there is known a core material having a significantly small core loss (see Patent Document 2; U.S. Pat. No. 7,132,019 (Japanese Unexamined Patent Application Publication No. 2005-307291)). In an alloy represented, for example, by a composition of Fe76.4Cr2.0P10.8C2.2B4.2Si4.4, good characteristics are obtained, i.e., a core loss of 250-380 kW/m3 at 100 kHz and 0.1 T and relative permeability μ of 36.8-37.1 in a DC magnetic field of 5500 A/m in a frequency range until 1 MHz.
As one of techniques for providing a satisfactory DC current characteristic in a large-current region (high-field region)) without causing saturation of magnetic flux in a core, there is known a technique of a mixing magnetic powder and a resin, i.e., a nonmagnetic powder, with each other (see Patent Document 3; Japanese Unexamined Patent Application Publication No. 2005-354001). With the known technique, 20% by volume, preferably, 40% by volume of resin is mixed to a Fe—Si alloy so as to suppress saturation of the relative permeability μ in a high magnetic field.
General soft magnetic iron alloys, such as a FeNi alloy, a Fe—Si alloy, and a Fe—Al—Si alloy, have relatively low electrical resistivity and therefore tend to generate a large eddy-current loss. In order to avoid an increase of the core loss caused by the large eddy-current loss and to obtain a good core loss characteristic, there is also known a technique of mixing a nonmagnetic insulating material, e.g., a resin, to the soft magnetic iron alloy to increase an electrical resistance value, thus improving the core loss characteristic (see Patent Document 4; U.S. Pat. No. 6,284,060 (Japanese Unexamined Patent Application Publication No. H11-238613) and Patent Document 5; U.S. Pat. No. 4,543,208 (Japanese Unexamined Patent Application Publication No. S59-119710 and No. S60-16406)).
However, when a gap is formed in a magnetic path as in the related art, apparent permeability can be reduced, but magnetic flux leaks through the gap, thus resulting in an increase of a core loss including an iron loss and a copper loss. Also, in an application such as a step-up coil in hybrid cars, a further reduction of permeability is required because of the necessity of supplying a large current flow. If the gap is formed in the magnetic path in such an application requiring the supply of a large current flow, mechanical strength is reduced and vibrations are generated due to attraction between magnetic bodies with the gap formed between them. In addition, noise is generated due to the vibrations.
When the amorphous soft magnetic iron alloy, e.g., the alloy represented by the composition of Fe76.4Cr2.0P10.8C2.2B4.2Si4.4 (Patent Document 2), is used in a region of large current (i.e., in an application where a current is 100 A or more and a generated magnetic field is 10000 A/m or more), the gap is required to be formed in the magnetic path. In that application, a problem occurs in practical use in that noise is generated due to vibrations near the gap formed in the magnetic path. By using the amorphous soft magnetic iron alloy, however, a good core loss characteristic of 250-380 kW/m3 is obtained in a region of not so large current (i.e., in an application where a current is 100 A or less and a generated magnetic field is 10000 A/m or less). Accordingly, there is no need to mix the nonmagnetic insulating material to increase the electrical resistivity as described in Patent Documents 4-5. In an embodiment described in Patent Document 4, the core loss characteristic is 476-1950 kW/m3 even with mixing of the nonmagnetic insulating material and is inferior to the core loss characteristic of the amorphous soft magnetic iron alloy described in Patent Document 2.
In the structure (Patent Document 1) in which the gap is filled with, e.g., a nonmagnetic body to maintain sufficient strength in a portion around the gap, the man-hours needed in the manufacturing process are increased and the cost is pushed up. Also, just simply filling the gap with, e.g., a nonmagnetic body is not a sufficient measure against the noise and a further improvement of the antinoise measure is required for practical use.
With the technique (Patent Document 3) of mixing the soft magnetic iron alloy and resin with each other to control saturation at a large current, the resin is mixed at a high ratio of 20% by volume or more, thus resulting in a restriction on annealing temperature. Another disadvantage is that the mixed material is susceptible to changes of resin components between before and after the annealing and to characteristic changes during a severe heat resistance test. In other words, the mixed material has various problems when used as materials of cores for use in products which are required to have heat resistance under severe applications, such as a reactor in hybrid cars.